Enhancing the stereoselectivity of Me2GaOR(NHC) species in the ring-opening polymerization of rac-lactide, with the help of the chelation effect

Abstract

Using dialkylgallium alkoxides with N-hetrocyclic carbenes (Me₂GaOR(NHC)) for the ring-opening polymerization of rac-lactide, we have demonstrated the effect of the chelate interaction between the growing PLA chain and gallium on the stereoselectivity of dialkylgallium alkoxide propagating species – Me₂Ga(OPLA)(NHC). In order to do so, we have conducted the structure–activity studies of both Me₂Ga(OCH₂CH₂OMe)(NHC) (NHC = SIMes (1) and IMes (2)) and Me₂Ga(OCH(Me)CO₂Me)(NHC) (NHC = SIMes (3) and IMes (4)), the latter mimicking active species in the ROP of lactide with growing PLA chain. Based on VT NMR and FTIR spectroscopy, the effect of toluene, CH₂Cl₂ and THF on the structure of 3 and 4 have been demonstrated, especially with regard to the interaction of methyl lactate ligand with gallium. In a combination with the latter, the studies on the activity of 1 and 2 in the ROP of rac-LA, in different solvents, and at temperatures between −40 °C and 40 °C, have shown the extent of the chelation effect on the isoselectivity of Me₂Ga(OPLA)(NHC) in the ROP of rac-LA, which varied between P_m of 0.75 and 0.89 depending on the polymerization conditions. Both the latter, and the contribution resulting from the structure of Me₂Ga(OPLA)(NHC) (P_m = 0.75) have been decisive for the total isoselectivity observed under specific conditions. Our finding represents the first evidence demonstrating that the chelation effect, resulting from the weak interaction between the growing PLA chain and the metal centre, can be responsible for the enhancement of stereoselectivity in the ROP of rac-LA with metal alkoxide propagating species. It should remain of interest, especially in the case of metal based catalysts, which are able to carry out the stereoselective polymerization of rac-LA at mild conditions, under which the chelation effect can manifest itself.